Composite circuits including a normally-on transistor device and a normally-off transistor device connected in series are known. In this type of composite circuit, a normally-on transistor device with a high-voltage blocking capability and a low on-resistance may be combined with a low-voltage normally-off transistor in order to obtain a normally-off switching behavior of the composite circuit.
According to one conventional concept, the normally-off transistor has a control terminal (gate terminal) coupled to one of the load terminals of the normally-on transistor. This circuit can be switched on and off by switching on and off the normally-off transistor device, because the switching state of the normally-on transistor always follows the switching state of the normally-off transistor device.
According to a further conventional concept, the normally-off transistor is permanently switched on when the circuit is in a normal operation mode, and the normally-on transistor device is switched on and off by virtue of a drive signal applied to its control terminal.